1. Field of the Invention
This invention relates generally to apparatus for measuring the flow of liquids or gases. More particularly, this invention relates to flowmeters of the vortex-shedding type, and to improvements in construction thereof.
2. Description of the Prior Art
It is well known in the art that vortices are developed in a fluid flowing past a bluff body or other suitable non-streamlined obstruction, and that with the use of certain configurations, the vortices shed at regular intervals from the edges of the obstruction to form a row of vortices, commonly referred to as a Von Karman "vortex street". Furthermore, it is well established that the spacing between successive vortices is essentially constant over a range of flow rates; therefore, the frequency of vortex formation is proportional to the velocity of the fluid. Thus, the fluid flow rate may be measured by sensing the frequency of vortex formation.
Various designs have been proposed for sensing the vortices downstream from the bluff body. A technique that has met with significant success is shown in U.S. patent application Ser. No. 639,776, filed by J. R. Curran et al on Dec. 11, 1975, and assigned to the same assignee as the present application. In that Curran application, a bar-like member having flat, parallel side surfaces is located downstream of the blunt front face of the vortex-shedding body. Within these side surfaces is formed an interior chamber containing a vortex sensor (e.g., a piezo-electric ceramic disc) sealed from the process fluid by flexile diaphragms at both side surfaces. The bar member is integrally joined to the vortex-shedding body such that the side surfaces of the bar-member are aligned with the direction of flow. This "shedder" apparatus thus comprises a bluff body with sharp side edges adapted to produce spaced rows of vortices and downstream thereof, a bar member housing the vortex sensor. The overall structure (i.e., shedder and sensor) forms part of an elongate, integral meter member which is insertable through an opening in the top of a pipe section for the purpose of measuring flow rate in the pipe.
Such prior design techniques in which the vortex sensor and the shedder are so physically interconnected as to form an integral unit have not been fully satisfactory in all applications. For example, when measuring the flow rate of corrosive liquids or gases, such configurations require that the entire metering structure be made from a non-porous, corrosive resistant material such as stainless steel. This type of construction results in increased manufacturing costs due in part to the cost of raw materials involved, and also due to the extensive machining required to shape the structure so as to effectively shed and detect vortices over a wide range of flow rates. In addition, such integral vortex-shedders/sensors have significant impact upon development costs associated with laboratory check-out of units prior to sale. Since the best way to measure the performance of the flowmeter is by testing completed units, the failure of a vortex sensor under test may, in some instances, require the discarding of the shedder together with the sensor, or, at the very least, extensive factory rework of the unit, including precision realignment of new components.
Also, an integral shedder/sensor unit generally requires a relatively large cut-out in the pipe wall to allow for the units insertion. This can create problems in effecting good pressure-tight sealing under certain conditions.
Still another factor is that, in most industrial instrumentation applications, field maintenance is becoming increasingly more significant to reduce outage of measurement capability. When field failures of an integral shedder/sensor vortex flowmeters occur, most often the device must be transported to a field maintenance depot for repair. Such depots may be a considerable distance from the installation site so that the affected unit may be away from its site for a prolonged period of time. Therefore, it is desireable to provide for rapid repair at the site, or quick replacement to minimize downtime.